DESCRIPTION

This module is about the numeric part of plain (string) Perl scalars. A scalar has a numeric value, which may be expressed in either the native integer type or the native floating point type. Many values are expressible both ways, in which case the exact representation is insignificant. To fully understand Perl arithmetic it is necessary to know about both of these representations, and the differing behaviours of numbers according to which way they are expressible.

This module provides functions to extract the numeric part of a scalar, classify a number by expressibility, and compare numbers across representations.

This module is implemented in XS, with a pure Perl backup version for systems that can't handle XS.

FUNCTIONS

Each "sclnum_" function takes one or more scalar numeric arguments to operate on. These arguments must be numeric; giving non-numeric arguments will cause mayhem. See "is_number" in Params::Classify for a way to check for numericness. Only the numeric value of the scalar is used; the string value is completely ignored, so dualvars are not a problem.

Decomposition

scalar_num_part(SCALAR)

Extracts the numeric value of SCALAR, and returns it as a pure numeric scalar. The argument is permitted to be any scalar.

Every scalar has both a string value and a numeric value. In pure string scalars, those resulting from string literals or string operations, the numeric value is determined from the string value. In pure numeric scalars, those resulting from numeric literals or numeric operations, the string value is determined from the numeric value. In the general case, however, a plain scalar's string and numeric values may be set independently, which is known as a dualvar. Non-plain scalars, principally references, determine their string and numeric values in other ways, and in particular a reference to a blessed object can stringify and numerify however the class wishes.

This function does not warn if given an ostensibly non-numeric argument, because the whole point of it is to extract the numeric value of scalars that are not pure numeric.

Classification

sclnum_is_natint(VALUE)

Returns a truth value indicating whether the provided VALUE can be represented in the native integer data type. If the floating point type includes signed zeroes then they do not qualify; the only zero representable in the integer type is unsigned.

sclnum_is_float(VALUE)

Returns a truth value indicating whether the provided VALUE can be represented in the native floating point data type. If the floating point type includes signed zeroes then an unsigned zero (from the native integer type) does not qualify.

Comparison

sclnum_val_cmp(A, B)

Numerically compares the values A and B. Integer and floating point values are compared correctly with each other, even if there is no available format in which both values can be accurately represented. Returns -1, 0, +1, or undef, indicating whether A is less than, equal to, greater than, or not comparable with B. The "not comparable" situation arises if either value is a floating point NaN (not-a-number). All flavours of zero compare equal.

This is very similar to Perl's built-in <=> operator. The only difference is the capability to compare integer against floating point (where neither can be represented exactly in the other's format). <=> performs such comparisons in floating point, losing accuracy of the integer value.

sclnum_id_cmp(A, B)

This is a comparison function supplying a total ordering of scalar numeric values. Returns -1, 0, or +1, indicating whether A is to be sorted before, the same as, or after B.

The ordering is of the identities of numeric values, not their numerical values. If floating point zeroes are signed, then the three types (positive, negative, and unsigned) are considered to be distinct. NaNs compare equal to each other, but different from all numeric values. The exact ordering provided is mostly numerical order: NaNs come first, followed by negative infinity, then negative finite values, then negative zero, then unsigned zero, then positive zero, then positive finite values, then positive infinity.

In addition to sorting, this function can be useful to check for a zero of a particular sign.

BUGS

In Perl 5.6, if configured with a wider-than-usual native integer type such that there are native integers that can't be represented exactly in the native floating point type, it is not always possible to distinguish between integer and floating point values in pure Perl code. In order to get the full benefit of either type, one is expected (by the numeric semantics) to know in advance which of them one is using. The pure Perl version of this module can't operate on such a system, but the XS version works fine. This problem is resolved by Perl 5.8's new numeric semantics.

Perl doesn't consistently maintain the distinction between integer and floating point zeroes, where the latter are signed. Zeroes of one type are liable to be mutated into another type by being used as operands in numeric operations. From Perl 5.8 onwards, weird zeroes can arise that don't consistently behave as any kind of pure numeric zero. The functions of this module are always careful to avoid causing side effects on their arguments. If given a weird zero, the functions of this module will treat it as some kind of pure numeric zero, but there is no guarantee of which kind it will be treated as.